The Consultative Committee on International Telegraph and Telephone (CCITT) sets international telecommunications definitions and standards for four categories of facsimile equipment (Groups 1, 2, 3 and 4). Most recently, Group 3 (G3) facsimile equipment have become popular because they enable a document page to be transmitted by a digital modem over a telephone-type circuit in one minute or less by employing digital-data-compression techniques. Detailed standards for G3 equipment are found in recommendations T.4 and T.30 of CCITT.
A typical facsimile transmission is shown in FIG. 1, wherein a transmitting facsimile machine 1 includes a scanner 1a for scanning a document and for providing information representing the scanned document at a rate which is equal to or less than 9.6 kbit/s, i.e., scanner 1a outputs baseband data. The facsimile machine 1 further includes a modem 1b which receives the baseband data output from the scanner 1a and which functions to convert the baseband data to analogue voiceband data. Specifically, modem 1b serves to convert and shift the spectrum of the digital baseband upwards in frequency so as to output analogue voiceband data which is carried by analogue communication line 6 to the digital circuit multiplication equipment (DCME) transmitter device 3 via analogue-to-digital (A/D) converter 3a. The DCME transmitter 3 transmits digital voiceband data at rates of 16, 24, or 32 kbit/s signals carrying voice traffic, or 40 kbit/s carrying non-voice or data traffic, e.g., facsimile data. The digital voiceband link may be a digital satellite or terrestrial cable communication line for connecting the DCME transmitter 3 and the DCME receiver 4.
The DCME receiver 4 and facsimile machine 2 perform the reverse operations of DCME transmitter 3 and facsimile machine 1, respectively. Specifically, digital facsimile data received by receiver 4 is converted into analogue facsimile data via D/A converter 4a for input to facsimile machine 2. The facsimile machine 2 includes a modem 2a which shifts the spectrum downward and converts and data to 9.6 kbit/s baseband data or less for input to a printer 2b which reproduces the document originally scanned in scanner 1a of facsimile machine 1.
As discussed in U.S. application Ser. No. 07/720,990, entitled "FACSIMILE COMMUNICATION SYSTEM," filed Jun. 26, 1991, which is expressly incorporated herein by reference, facsimile transmissions have been used effectively in mobile communication systems wherein the DCME 3 of FIG. 1 is replaced by a mobile telephone switching office (MTSO) or a fixed earth station (FES), and the DCME 4 is replaced by a mobile terminal (MT) or a mobile earth station (MES) (as shown in FIG. 2). In the figure, the system 50 includes two facsimile machines 52 and 54, two FIUs 56 and 58, and a digital transmission channel 60 for connecting the two FIUs 56 and 58. For facsimile messages originating from facsimile machine 52, FIU 56 is the demodulating FIU for converting the analogue voiceband data to digital baseband data, FIU 58 is the remodulating FIU for converting the digital baseband data to analog voiceband data, and facsimile machine 54 is the receiving facsimile machine. The digital channel 60 is composed of a single 10.5 kbit/s channel, i.e., a channel having much lower band width than that used by a DCME.
By transmitting G3 facsimile signals over digital channels in their baseband, as in the systems described above, substantial transmission gain and quality enhancement advantages can be attained. The advantages are attributed to the fact that using digital channels reduces the analogue links required. However, the above systems often require additional out-of-band transmission capacity that is used to carry information concerning the type of traffic (called "line state information").
U.S. Application Ser. No. 07/754,785, entitled "LINE STATE TRANSMISSION SYSTEM OVER DIGITAL CHANNELS FOR GROUP 3 FACSIMILE," filed Sep. 4, 1991, improves upon the above systems by providing a facsimile communication system which permits line state information to be transmitted in-band together with user-data without the need for additional out-of-band transmission capacity. In particular, signals used for link equalization (e.g., training and synchronization) can be regenerated by the modulating FIU when signaled to do so by the demodulating FIU. In this manner, not all of the signals need to be transmitted over the digital channel. To permit the selection of a suitable modulator and/or the precise timing for the regeneration of link equalizing signals, the demodulating FIU must generate appropriate control signals and transmit these control signals to the modulating FIU in order to convey the line state (LS) changes and their corresponding timing relationships.
For example, as shown in FIG. 3, initial idle line state information I is being transmitted over the digital channel by a sending FIU such as FIU 56 (FIG. 2). The sending FIU 56 then detects a line state transition at time T1 (i.e., a transition from the idle state to a synchronization state). Upon this detection, the sending FIU 56 assembles a digital line control packet 30A having preamble P and S synchronizing signal connection line transition code SSC. After this line control packet is transmitted to the remodulating FIU, the synchronizing data S is transmitted over the digital channel. This data S continues to be sent over the digital channel until time T2 at which time another transition is detected by the demodulating FIU (i.e., a transition from the synchronization state to a training state TCF). By transmitting the line control packet in-band, the control signals used out-of-band in the previously described systems are no longer needed.
A number of mobile systems, including the European digital mobile radio, the International Maritime Satellite organization (INMARSAT) standard-B and standard-M, the aeronautical INMARSAT, and the AUSSAT facsimile standards, employ FIUs to provide the necessary voiceband-to-baseband and T.30-to-satellite protocol conversions. As at least one of these systems (INMARSAT-M) is known to have link budgets which have been miscalculated, and therefore backward compatible enhancements to the current system specifications will be necessary. Although alternative devices for use as facsimile interface units may be designed, these devices may not enhance the capabilities of the systems while maintaining backward compatibility with the existing communications system specifications such as those for the INMARSAT system, or they may not be capable of further enhancements.